In recent years, as electronic apparatuses have become smaller in size, portable devices have been widely used that are driven by a battery pack having a rechargeable battery, which is chargeable and dischargeable. Conventional battery packs built in portable devices have a function of calculating the dischargeable capacity of a rechargeable battery and notifying the portable devices of the dischargeable capacity. In the case of calculating a dischargeable capacity in conventional battery packs, the dischargeable capacity is determined by periodically measuring a discharge current from the rechargeable battery at predetermined intervals and adding up the measured discharge currents.
However, if there is an error in the dischargeable capacity determined by adding up the discharge currents, the dischargeable capacity calculated on the battery pack side may not correspond to a discharge cutoff voltage, which is a minimum voltage necessary to drive the portable device.
In particular, since the rechargeable battery has the voltage characteristic of a sudden voltage drop at the last stage of discharging, the error in the dischargeable capacity increases at the last stage of discharging. Therefore, even if the dischargeable capacity calculated on the battery pack side indicates the capability of causing the portable device to operate, the actual voltage may have reached the discharge cutoff voltage. In this case, the portable device stops operating in response to the rechargeable battery in the battery pack reaching the discharge cutoff voltage. Therefore, the portable device may stop operating when the user least expects it.
Therefore, it has been desired to improve the accuracy of detecting the dischargeable capacity. For example, Patent Document 1 describes a rechargeable battery remaining capacity computing apparatus and method that compute the remaining capacity of a rechargeable battery at the last stage of discharging with accuracy.